1. Field of the Invention
The present invention relates to a process for producing a perfluoropolymer, a production apparatus and a process for producing an electrolyte membrane for polymer electrolyte fuel cells.
2. Discussion of Background
A perfluoropolymer is used in various fields such as coating of e.g. a chemical plant or a semiconductor production apparatus; covering of a cable or an optical fiber; and automobile components. Recently, a perfluoropolymer having —SO3H groups has attracted attention as a prospective material for an electrolyte membrane for polymer electrolyte fuel cells.
However, immediately after obtained by polymerizing a perfluoromonomer such as perfluorocarbon, a perfluoropolymer is not completely perfluorinated, and at some of its molecular chain terminals, it has unstable functional groups (hereinafter, referred to as unstable terminal groups) such as a —COOH group, a —CF═CF2 group, a —COF group and a —CF2H group. Accordingly, if a perfluoropolymer is used as en electrolyte for fuel cells for a long time, there is a problem that the perfluoropolymer is gradually decomposed by the presence of the unstable terminal groups, whereby the power generation voltage deteriorates. Further, there is a problem that the mechanical strength of the electrolyte membrane deteriorates as the perfluoropolymer is decomposed, and e.g. pinholes, cracks or delamination results, whereby it becomes impossible to use the fuel cell.
As a process for producing a perfluoropolymer having unstable terminal groups reduced, the following method has been proposed.
A method of introducing a 2 mm-thick sheet made of a copolymer of tetrafluoroethylene and CF2═CFOCF2CF(CF3)O(CF2)2SO2F into an autoclove reactor and fluorinating it at 190° C. for 4 hours (Patent Document 1).
However, such a method has the following problems:
(i) It takes a few hours to raise and lower the temperature of the autoclove reactor.
(ii) It takes a few hours to fluorinate the sheet to its center since it is thick.
(iii) Even if the sheet is made thin so as to shorten the time for fluorination, the amount of a perfluoropolymer which can be produced per treatment decreases.
(iv) In order to take out the perfluoropolymer from the autoclove reactor, it is necessary to carry out degasification of fluorine gas completely, but it takes a long time (about one day) to replace the interior of the autoclove reactor with nitrogen gas.
(v) Since a batch system is employed in the method, it is impossible to carry out fluorination treatment continuously.
In view of the problems of (i) to (v), the method as described in Patent Document 1 has a problem that it is impossible to efficiently produce a perfluoropolymer having unstable terminal groups reduced.
Patent Document 1: WO2004/102714 (EXAMPLE 1 on page 9)